Variability in Stem Methane Emissions and Wood Methane Production of Different Tree Species in a Cold Temperate Mountain Forest

Abstract

The role of trees, in addition to that of the soil, must be considered in CH₄ budget for forests. Although trees can emit CH₄ through their stems, there are uncertainties about the main factors that explain inter- and intraspecific variations, which impedes upscaling of measurements from the stem to the ecosystem level. This study aimed to characterize the variability in CH₄ emissions ($F_{{{\text{CH}}_{4} }}$) from stems between species and individuals, and within individuals. We measured $F_{{\rm CH}_{4}}$ in situ during the snow-free period in five species in a temperate mountain forest, using individuals of different sizes and chambers at different heights along the stems. One coniferous species emitted almost no CH₄, whereas four broadleaved species exhibited high intraspecific variability in $F_{{\rm CH}_{4}}$ (0–3.7 nmol m⁻² s⁻¹). Increasing trends in $F_{{\rm CH}_{4}}$ with tree diameter were observed for four species. The vertical patterns in $F_{{\rm CH}_{4}}$ were complex. Seasonal variations in $F_{{\rm CH}_{4}}$, measured on two trees per species, were well explained by air temperature with apparent temperature sensitivity coefficients (Q₁₀) between 1.2 and 2, which were not related to the antecedent precipitation indices, whether calculated over 7 or 30 days. Potential CH₄ production was detected in wood core segments incubated under anoxic conditions in the majority of individual trees of all species. Our results suggest that the CH₄ emitted by trunks can originate either from soil or internal sources. Scaling $F_{{\rm CH}_{4}}$ from trees at the stand level and developing process-based models of $F_{{\rm CH}_{4}}$ will remain challenging until the sources of variation are better explained.